nsproxy: add a cleanup helper for nsproxy

Add a simple cleanup helper for nsproxy.

Link: https://lore.kernel.org/r/20240627-work-pidfs-v1-2-7e9ab6cc3bb1@kernel.org


Reviewed-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>